Sound reproducing system



July 23, 1935. J. H. HAMMOND, JR 2,008,705

SOUND REPRODUCING SYSTEM Ofiginal Filed Janl 22, 1931 2 Sheets-Sheet 1 INVENTOR ATTORNEYS July 23, 1935. J. H. HAMMOND, JR 5 I SOUND REPRODUCING SYSTEM Original Filed Jan. 22, 1931 2 Sheets-Sheet 2 FIGHZ;

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OuT uT Decibels In pm! Decibels INVENTOR A TTORNE Y5 Patented July 23, 1935,

UNITED STATES PATENT OFFICE Application January 22, 1931, Serial No. 510,383 Renewed September 22, 1933 11 Claims. (CL 179-1001) This invention relates to a sound reproducing system, and more particularly to an improved system for automatically expanding or multiplying the dynamic range of the sound energy.

The invention provides more specifically for the simultaneous control of the gain ratio or amplification of a plurality of cascaded stages of amplification in accordance with the energy applied to the system whereby the volume ratio of the signal may be expanded in an improved manner.

The invention further provides for the use of push pull amplification. in either or both stages, and other arrangements to eliminate from the amplifier output any undesired signal that may pass through the control system when the input to the amplifiers'is small.

The invention also consists in certain new and original features of construction and combinations of parts hereinafter set forth and claimed.

Although the novel features which are believed to be characteristic of this invention will be particularly pointed out in the claims appended hereto, the invention itself, as to its objects and advantages, the mode of its operation'and the manner of its organization may be better understood by referring to the following description taken in connection with the accompanying drawings forming a part thereof, in which:

Fig. 1 illustrates diagrammatically the system as applied to the reproduction of the sound record of a talking moving picture projector or phonograph; and

Fig. 2 shows typical curves of relationship between input and output decibels attainable by this system. i

In the following description and in the claims, parts will be identified by specific names for convenience, but they are intended to be as generic in their application to similar parts as the art will permit.

Referring to the accompanying drawings, and more particularly to Fig. 1, a phonograph pick up I0 is shown which is connected to two contacts of a double-pole double-throw switch II, the other contacts of which are connected through a transformer I2 to a photoelectric cell I3. A lamp I4 is provided, the light from which is focused, by means of a lens I5, through an aperture I6 in an aperture plate I1, upon the sound record of a movingphotographic film I8. The light which passes through this film then impinges on the photoelectric cell I3.

The blades of the switch II are connected through a transformer 2| to a potentiometer 22,

which is connected in the input circuit of a push pull amplifier 24 comprising two space discharge devices I24. The output circuit of this amplifier includes the primary of a transformer 25 which is shunted by a resistance 26. The secondary of the transformer 25 is connected in the input circuit of a second push pull amplifier 21 comprising two space discharge amplifiers I21. The output circuit of this amplifier includes the primary of a transformer 28, which is shunted by a resistance 29. The resistances 26 and 29 are provided in order to maintain the. frequency characteristics of the circuits independent of the gain ratio characteristics. A meter 30 may be inserted in the plate circuit of the push pull amplifier 21. The secondary of the transformer 28 is connected to a power amplifier 3!, the output of which is connected to a plurality of loud speakers 32, 33.

It is obvious that either or both of the push pull amplifiers 24 and 21 could be replaced by a simple amplifier if desired.

It is to be understood that additional amplifiers may be inserted in the photoelectric pick up circuit, as for example between transformer I2 and switch II, if desired.

The potentiometer 22 is shunted by a bank of condensers 23 for controlling the tone quality. The secondary of the transformer 2I is connected through a stopping condenser 34 to the input circuit of a space discharge device 39, the output circuit of which is connected to the primary of a transformer 4|, the secondary of which is connected to a circuit including a rectifier 44 and resistances 45 and 45. Suitable filtering and timing condensers 41 and 48 are provided. A single pole double-throw switch 52 connects the grid return from the amplifiers to either an adjustable contact 53 on a resistor assembly 54 or to a bank of condensers 55. The switch 52 is used either to provide a suitable initial bias for the amplifiers from adjustable contact 53 or to connect the bank'of condensers 55 across the resistor 46 for the purpose to be pointed out.

The filaments of the devices I24, 39, I21 and 44 are supplied with current from the secondaries of a transformer 56, the primary of which is connected to an A. C. source 51.

Suitable sources of plate power and grid bias voltage are shown as batteries and 6|, with filter condensers 52 and 63, but it is understood that these may be replaced by any suitable source.

Tubes I24 and I21 are supplied with plate power from the most positive point of battery 60, and cathodes and center point of filaments are suitably arranged with common connections, as well as the grid return connections, connected to junction of resistors 45 and 46. The auxiliary amplifier 39, however, receives plate current through resistor 64, from which tone output from tube 39 is excluded by condenser 65. The grid return, cathode and filament connections of this tube are suitably chosen for normal amplification by this tube. This type of connection may be used to limit the amount of D. C. power available for operating the amplifier tube 39, and in turn to limit the change of bias upon the tubes I24 and I21 due to current flowing through resistance 46.

An adjustable contact 58 on a resistance 59 is used for providing the bias at low signal input for devices I24 and I21.

In the operation of the form of the invention shown in Fig. 1, with no applied signal, the voltage on the grids of the push pull amplifiers 24 and 21 is determined by the setting of the adjustable contact 58 on resistance 59, which is so positioned that small plate current will fiow through the push pull amplifier 21, as indicated by the reading of the meter 30.

When a signal is received from either the phonograph pick up In or the photoelectric cell I3, energy will pass through the transformer 2I to the potentiometer 22 which controls the amount of energy fed to the input circuit of the push pull amplifier 24. The output of this amplifier is passed through the transformer 25 to the input circuit of the push'pull amplifier 21. Part of the energy passing through the transformer 2I is applied to the input circuit of the control device 39. The output of this device passes through the transformer 4| and is rectified by the rectifier 44. This rectified current passes thru the resistance 45 and through resistance 46 in the direction of the arrows, thus causing a potential difference to be built up across the resistance 46 which is an increasing function of the input signal strength from transformer 2I. This potential difference will oppose the biasing voltages of the amplifier tubes I24 and I21, as determined by the setting of the contact 58, thus decreasing the bias on these devices, which increases the amplification factors or gain ratios of stages 24 and 21 of which these devices I24 and I21 form a part.

- This is accompanied by an increase of plate ourrent of the tubes I24 and I21, the latter being indicated by the meter 30.

In this invention, potentiometer 22 provides for adjusting the signal to amplifier tubes I24 independently of the means for actuating the control amplifier tube 39, which through the agency of rectifier 44 supplies current through resistance 46 for changing the bias on tubes I24. This permits the volume of signal through tubes I24 to be any suitable value that may be desired, as for instance strong or weak, without alteration of the means for changing the dynamic amplification properties Since, however, the potentiometer 22 may be set low, it is especially desirable to reduce to a minimum the effect of any tone voltage across resistance 46, residual after rectification by rectifier tube 44. This residual tone may be made small by use of a sufllciently large by-passcondenser, but since this increases the time lag, it has been found more desirable to split the by-pass condensers into two parts 41 and 48 with a resistance 45 between them. In this manner improved filtering is accomplished with small amount of increase of timing.

In addition, by using complete push pull tube amplifier arrangements, any tone voltage across resistance 46 is inserted into the grid circuits of the tubes in such a manner that amplified tone voltages from across resistance 46 are not passed on to succeeding stages. Thus in addition to the use of push pull circuits for obtaining improved power and quality, theiruse in this dynamic multiplier arrangement permits elimi nation of distorted tone signals residual in resistance 46 from passing through the amplifiers and permits an especially low time constant to be used for the rectifier output circuit.

The output of the push pull amplifier 21 passes through the transformer 28 to the input circuit of the high power amplifier 3|, there being amplified and fed to the loud speakers 32 and 33, where it is reproduced as speech or music in the usual manner.

When the system is to be used chiefly for speech, the switch 52 is left open, and when used for music this switch is thrown down, thus throwing the condensers 55 across the resistance 46. By selecting the proper condenser in the bank 55 it is possible to vary the rate of operation of the dynamic multiplier, i. e. by increasing the capacity across the resistance 46, the actio of the dynamic multiplier is retarded.

If it is desired to eliminate the dynamic multiplier entirely, the switch 52 is thl'OWlLLlD, this applying a small negativevoltage from the adjustable contact 54 to the grids of the amplifier tubes I24 and I21 which then operate as normal amplifiers.

The operation of this circuit is further illustrated by curves of Fig. 2, showing the relation between the output and input decibels. The straight line H depicts the usual condition of non-dynamic multiplication, with the switch 52 thrown up to give normal bias for the tubes I24 and I21 for which the output to the amplifier 3| covers the same decibel range, namely 36 decibels, as the input from the secondary of the transformer 2|. The curved line 12 depicts dynamic multiplier operation, in which the setting of potentiometer 22 is the same as for the curve H, but the sound record feeding the transformer 2| is somewhat stronger than the average sound records for which the control circuit is adjusted, so that its strongest output reduces the bias on the tubes I24 and I21 to a value somewhat less than the fixed value attained when the switch 52 is thrown up, to which condition curve 1I pertains.

The straight line 13 shows a relation of input and output decibels for 24 decibels even expansion, with three decibels change of input producing five decibels change of output, irrespective of whether the signals are low or high.

In the operation of the present invention, change of setting of potentiometer 22 does not alter the input decibels delivered by the pick-up devices but increases the output decibels by an equal amount for all pick-up inputs, so that the relationship between changes of output and input decibels, that is, decibel range, is maintained irrespective of the actual value of output.

While certain novel features of the invention have been disclosed and are pointed out in the annexed claims, it will be understood that various omissions, substitutions and changes may be made by those skilled in the art without departing from the spirit of the invention.

What is claimed is: v

1. In a system for reproducing sound from a sound record comprising a plurality of space discharge amplifiers having variable gain ratio characteristics, a control circuit for controlling said characteristics, means for operating said control circuit in accordance with .the average signal strength and means for preventing signals from passing through said control circuit including a filter comprising a pair of parallel condensers connected across said control circuit and separated by a series resistance.

2. In a system for reproducing sound from a sound record, a plurality of stages of amplification connected in cascade, each stage comprising space discharge amplifiers arranged in push-pull relationship, a control circuit for controlling the amplification factor of each of said stages, said control circuit being operable in accordance with the average value of the signal strength, said control circuit being associated with said pushpull amplifiers in such manner that tone signals are prevented from passing from said control circuit to the output of said amplifiers.

3. A system for reproducing sound from a sound record comprising a plurality of stages of amplification connected in cascade, each stage comprising a pair of space discharge amplifiers arranged in push-pull relationship, an impedance connected in the common grid returns of both of said stages and means for passing a current through said impedance proportional to the average value of the input signal strength in such direction that the voltage drop therein opposes the normal biasing voltage of said space discharge devices.

t. In a system for reproducing sound from a sound record, a pair of space discharge amplifiers arranged in cascade, each of said amplifiers including a pair of space discharge devices arranged in push-pull relationship, control means for varying the amplification factor of both of said stages and means for operating said control means in accordance with the strength of signal applied to the first of said stages and means for varying the intensity of signal applied to said amplifiers independently of the amplification factors thereof.

5. In a sound reproducing system, a soundrecord, a pick-up device therefor, an amplifier fed by said pick-up device comprising a plurality of push-pull stages, a control system for controlling the gain ratio of all of said stages, and means in said control system for rectifying and averaging the signal impressed thereon originating in said pick-up device.

6. In a sound reproducing system, a sound record, a pick-up device therefor, an amplifier fed by said pick-up device, a control circuit fed by said pick-up device having a biasing resistance for controlling the gain ratio of said amplifier, means in said control circuit for rectifying and averaging the current including said resistance, a first capacitance across said resistance, a second capacitance and a second resistance serially connected across said first capacitance and biasing the input signal'strength insuch direction that the voltage drop therein opposes the normal biasing voltage of said space discharge devices.

8. The method of reproducing a sound record which comprises amplifying the energy of said record in a push-pull amplifier and controlling the amplification ratio of said amplifier in accordance with the input energy.

9. In a system for reproducing sound from a sound record, a pair of space discharge amplifiers arranged in cascade, each of said amplifiers including a pair of space discharge devices arranged in push-pull relationship, a single control means for varying the amplification ratio of both of said stages, and means for operating said control means in accordance with the strength of signal applied to the first of said stages.

10. In a. system for reproducing sound from a sound record, a plurality of stages of amplification connected in cascade, each stage comprising space discharge amplifiers arranged in pushpull relationship, and a single means to control the amplification ratios of each of said stages in accordance with the average signal strength.

11. Ina sound reproducing system, a sound record, a pick-up device therefor, an amplifier fed by said pick-up device comprising a plurality of push-pull stages, means for rectifying the signal to produce a sub-audio control frequency,

and means for utilizing said control frequency to control thegain of a plurality of said stages. JOHN HAYS 0ND, JR. 

